Temperature resistant material comprising short metal fibers

ABSTRACT

A temperature resistant material, comprising a temperature resistant matrix and a set of short metal fibers, which characterized in that the set of short metal fibers represents at least 0.5% by weight of the temperature resistant material. The set of short metal fibers has an equivalent diameter D in the range of 1 to 150 μ, and comprising curved fibers and entangled fibers. The curved fibers have an average length L in the range of 10 to 2000 μ.

FIELD OF THE INVENTION

The invention relates to temperature resistant materials such as ceramicmaterial, ceramic glue or temperature resistant glue comprising shortmetal fibers. The invention further relates to a method to improvethermal shock resistance of temperature resistant glues and ceramicmaterials.

BACKGROUND OF THE INVENTION

High temperature resistant glues, e.g. ceramic glues, are known in theart. Such glues have in general the disadvantage to become relativelybrittle after an exposure to (too high) temperatures, or degrade in timewhen subjected to thermal shocks, showing brittleness and small cracks.Ceramic materials and structures, being resistant to increasedtemperatures, also become brittle and show small cracks due to thermalshocks with relatively important temperature differences.

Further, it is known to add metal powder to glues to render the glueelectro-conductive properties.

Short metal fibers in general are known in the art.

Metal fibers having a rather flat cross section, with diameter less than15 μm and a length of less than 400 μm are known from U.S. Pat. No.4,703,898.

These fibers have a crescent shape and have a small, point-like hook atboth ends.

JP2175803 describes similar short metal fibers, which have a curvedshape.

Short metal fibers are also known from GB889583. These metal fibers maybe undulated or “kinked” over their length. In this document, theseterms mean that the major axis of the fibers change two or more timesover the length of the fiber.

SUMMARY OF THE INVENTION

According to the present invention, a temperature resistant material,such as a ceramic material, ceramic glue or temperature resistant glue,comprises a temperature resistant matrix and a set of short metalfibers. The set of short metal fibers represents at least 0.5% of weightof temperature resistant material.

The temperature resistant matrix, used to provide a temperatureresistant material, is preferably ceramic material, ceramic glue ortemperature resistant glue. Preferably, ceramic matrices or ceramicglues based on SiO₂, Al₂O₃, ZrO₂ and/or MgO are used.

A set of short metal fibers used to provide the temperature resistantmaterial as subject of the invention is characterized by the presence oftwo different groups of short metal fibers, being “entangled” fibers and“curved” fibers.

A set of short metal fibers as subject of the invention comprises shortmetal fibers with an equivalent diameter “D” between 1 and 150 μmpreferably between 2 and 100. Most preferably the equivalent diameterranges between 2 and 50 μm or even between 2 and 35 μm such as 2, 4,6.5, 8, 12 or 22 μm.

With the term “equivalent diameter” is meant the diameter of animaginary circle, which has the same surface as the surface of a fiber,cut perpendicular to the major axis of the fiber.

The set of short metal fibers comprises entangled fibers. The number ofentangled fibers in a set of short metal fibers as subject of theinvention ranges from 5 to 35%. Preferably more than 10% of all shortmetal fibers in the set of short metal fibers are entangled. Thesefibers are hereafter referred to as “entangled fibers”. To have astatistically reliable percentage, a sample of at least 50 fibers,randomly chosen out of the set of short metal fibers are to beevaluated.

The percentage of entangled fibers is measured and calculated as:% entangled fibers=100×(#entangled/#total)wherein

#entangled=number of entangled fibers out of the sample;

#total=number of fibers out of the sample.

The entangled fibers of the set of short metal fibers as subject of theinvention have an average length “Le”, which is considerably longer asthe average length of the curved fibers “Lc”. The average length of theentangled fibers is at least 5 times the average length of the curvedfibers. Preferably, the average length of the entangled fibers is morethan 10 times the average length of the curved fibers. Preferably, theaverage length of the entangled fibers is larger than 200 μm, or evenmore than 300 μm, most preferably more than 1000 μm. The entangledfibers may be entangled with themselves (individually) or may beentangled together with some other entangled fibers. The entangledfibers, either individually or together with other entangled fibers,cannot be individualized as an essentially straight fiber out of theshape, which is defined by the entanglement of the fibers. The majoraxis of each fiber changes so often and unpredictably, that the fibermay be entangled in many different ways. Some of the fibers are presentin a shape, which resembles to a clew. The effect is comparable to theso-called pilling effect, well known in the textile industry, and incarpet industry more in particular. One or more fibers get trapped intoa small ball. The trapped fibers may not be separated from this ballanymore. Other fibers look more like a pigtail. They are characterizedby a major axis which changes several times in an unpredictable way, soa relatively chaotic shape may be provided.

The other short metal fibers out of the set of short metal fibers arehereafter referred to as “curved” fibers.

The average length “Lc” of the curved fibers of the set of short metalfibers may range from 10 to 2000 μm, preferably from 30 to 1000 μm suchas 100 μm, 200 μm or 300 μm. When a length distribution is measured fromthese curved fibers as part of a set of short metal fibers as subject ofthe invention, a gamma-distribution is obtained. This gamma-distributionis identified by an average length Lc and a shape factor “S”. Accordingto the present invention, the gamma-distribution of the length of thecurved fibers, has a shape factor S ranging between 1 and 10.

For average lengths Lc larger than 1000 μm, usually a shape factor Slager than 5 is measured. For average lengths Lc between 300 μm and 1000μm, a shape factor S between 2 and 6 is usually measured. For averagelengths Lc smaller than 300 μm, usually a shape factor S smaller than 3is measured. To have a statistically reliable distribution, at least 50curved fibers, randomly chosen out of the set of short metal fibers areto be measured.

The L/D ratio of a set of short metal fibers as subject of the inventionhas an L/D-ratio of more than 5, preferably more than 10, wherein L isthe average length of all fibers, present in a representative sample offibers from the set of short metal fibers. As described above, thissample comprises at least 50 fibers out of the set of short metalfibers. Preferably, but not necessarily, the curved fibers out of a setof short metal fibers as subject of the invention has an Lc/D-ratio ofmore than 5, preferably more than 10.

Further, a majority of these curved fibers have a major axis, whichchanges over an angle of at least 90°. This angle is the largest anglewhich can be measured between two tangents of this major axis.Preferably, 40% of the curved fibers has a major axis, changing morethan 900, e.g. more than 45%, or preferably more than 50%. To measurethese curves of the major axis, a microscopic image with appropriateenlargement is taken from several short metal fibers. Using a computerimaging system, the tangents of the major axis and the largest anglebetween them is calculated. To have a statistically reliable sample, atleast 50 curved fibers, randomly chosen out of the set of short metalfibers are to be measured.

A blend of short metal fibers and ceramic matrix or ceramic or hightemperature resistant glue, comprising up to 15% or even 20% by weightof short metal fibers, seems to resist thermal expansions to a largerextend, compared to the pure ceramic or high temperature resistant glue,once the glue or matrix comprising short metal fibers are cured. Ahigher resistance to thermal cracks in the glue was obtained. Thesepositive results were obtained especially when a set of short metalfibers is used which comprises entangled and curved fibers of which morethan 10% of the set of short metal fibers are individually entangledfibers.

Surprisingly, only a relatively small change in electrical conductivitywas noticed when the amount of the set of short metal fibers is keptlower than 10% by weight of the temperature resistant material, e.g. inthe range of 1% to 9.5%, in the mean time providing sufficientresistance to thermal shocks and cracks. Higher percentages by weight ofa set of short metal fibers may be used, e.g. more than 15% or even morethan 20% or 30%, however such percentages of weight are not absolutelynecessary to obtain a sufficient resistance to thermal shocks.

Short metal fibers having curved and entangled fibers may be obtained bya method comprising the following steps. First, metal fibers, beingpresent in a bundle of fibers, in a yarn or a textile structure, or evenas staple fibers, are individualized to some extend by a cardingoperation.

These more or less individualized fibers are brought into a comminutingdevice. In this device, each fiber is cut into short metal fibers byfast rotating knifes. The blade of these knifes, having a certain bladethickness, encounter or ‘hit’ the fibers usually in radial direction.The fibers are mechanically plastically deformed and entangled orpossibly broken into a smaller length. Due to the centrifugal force, theso provided short metal fibers (curved or entangled) are blown outwardlyagainst the external wall of the comminuting device. This external wallcomprises a sieve with well-defined openings. According to theseopenings, short metal fibers with a certain length may pass through thesieve, whereas too long short metal fibers will stay in the comminutingdevice and possibly be hit once again, until the lengths aresufficiently small to pass the sieve, or until they are entangled enoughto allow passage through the sieve.

According to the specific use of the short metal fibers, differentmetals and/or alloys may be used to provide the short metal fibers. Thealloy of the metal fibers is to be chosen in order to provide requiredproperties such as temperature resistance or electrical conductivity.Stainless steel fibers out of AISI 300-type alloys, e.g. AISI 316L orfibers based on INCONEL®-type alloys such as INCONEL®601 orNICROFER®-type alloys such as NICROFER® 5923 (hMo Alloy 59) and NICROFER6023, or fibers based on Fe—Cr—Al alloys may be used. Also Ni-fibers,Ti- fibers, Al-fibers, Cu-fibers or fibers out of Cu-alloy or otheralloys may be used.

Metal fibers may e.g. be bundle drawn or shaved, or provided by anyother process as known in the art.

The set of short metal fibers and the temperature resistant matrix areblend using techniques as known in the are. Usually, a curing procedureis to be followed after the application of the uncured temperatureresistant material, especially in case the temperature resistantmaterial is a temperature resistant glue.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference tothe accompanying drawings wherein

FIGS. 1A, 1B, 1C, 1D, 1E and 1F are images of short metal fibers, allbeing part of a set of short metal fibers comprising curved andentangled short metal fibers.

FIG. 2 shows a curved fiber being part of a set of short metal fibers assubject of the invention.

FIG. 3 shows a graph of the length distribution of a set of short metalfibers as subject of the invention.

FIG. 4 shows a graph of the curvature distribution of the curved fibersout of a set of short metal fibers as subject of the invention

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

A preferred embodiment of a set of short metal fibers is shown in FIGS.1A, 1B, 1C, 1D, 1E and 1F, which all show short metal fibers out of thesame set of short metal fibers. The short metal fibers, having anequivalent diameter of 22 μm, are obtained by providing a bundle of AISI316L bundle drawn fibers of a carding device and further to acomminuting device. As may be seen from FIGS. 1A to 1F, the shape of theshort metal fibers may be very different. Some short metal fibers areclearly entangled fibers, such as fibers 11, 12 and 13. Fibers 12 aremore curled irregularly, providing a non-defined shape. Fibers 13 areindividually entangled to a non-defined shape. Fibers 11, 12 and 13 areto be understood as “entangled fibers”. Other fibers 14 are clearlycurved, although the curving angles are unpredictably. Some curvedfibers, such as fiber 15, may have a limited curvature. An example ofsuch a curved fiber is shown schematically in FIG. 2. A curved fiber hastwo ends, being a first end 21 and a second end 22. A major axis 23connects the center of the transversal cuts over the whole length of thefiber. The direction of the major axis 23 changes over an angle α. Anglea is absolute value of the largest angle which can be measured betweentwo vectors 24 having a direction equal to the tangent of the majoraxis, starting point being a point of the major axis, and a sensepointing from first end 21 to second end 22.

FIG. 3 shows the angle distribution of the change of major axis of thecurved fibers of the set of short metal fibers from FIGS. 1A to 1F. Asample to 316 fibers, randomly chosen out of the total set of shortmetal fibers was taken. Each bar 33 in the graph represents the numberof fibers (to be read at the left ordinate 34), having a major axischanging with an angle α, α being smaller than the angle valueunderneath the bar, which is related to that bar, but larger than theangle, related to the bar at its left side. E.g. the bar related to 90°,indicates the number of curved fibers, having an angle a smaller than90°, but larger than 80°. Related numbers are summarized in Table I

TABLE I % curved with angle number angle α or α of in % curved withangle entangled/total fibers sample α/total curved fibers fibers 0 00.00 0.00 10 2 0.65 0.55 20 3 0.97 0.83 30 10 3.24 2.77 40 16 5.18 4.4350 16 5.18 4.43 60 19 6.15 5.26 70 22 7.12 6.09 80 21 6.80 5.82 90 185.83 4.99 100 17 5.50 4.71 110 10 3.24 2.77 120 14 4.53 3.88 130 14 4.533.88 140 15 4.85 4.16 150 28 9.06 7.76 160 18 5.83 4.99 170 31 10.038.59 180 35 11.33 9.70 entangled 52 — 14.40 total 52 entangled totalcurved 309 total 361

Line 31 indicates the cumulative curve of the number of curved fibershaving an angle α, less than the angle value in abscissa. This number isexpressed, as indicated on the right ordinate 35, in percentage comparedto the total number of curved fibers in the sample. More than 50% of thecurved fibers have a major axis direction changing more than 90°.

As also indicated in FIG. 3, more than 10% of all short metal fibers outof the set of short metal fibers are entangled fibers. This is indicatedby the dots 32, which represent the percentage of fibers, also to beread on the right ordinate 35, comprised in the related bar 33, comparedto the total number of short metal fibers out of the sample taken fromthe set of short metal fibers.

FIG. 4 shows the length distribution of the curved fibers of two sets ofshort metal fibers as subject of the invention.

A first length distribution 41, indicated with black bars, is a lengthdistribution of the curved fibers of a set of short metal fibers, havingan equivalent diameter of 8 μm. The set of short metal fibers wasprovided using bundle drawn stainless steel fibers, alloy AISI 302. Arepresentative and randomly chosen sample of 227 fibers was taken. A naveragelength Lc of 420 μm was found. The length distribution is agamma-distribution 42, being characterized with a shape factor S being3.05. The bars of distribution 41 is to be understood as the percentageof curved fibers out of the sample (read in ordinate 43), which has alength (expressed in μm and indicated in abscissa 44) in the range withupper limit as indicated underneath the bar, and lower limit being thelength indicated under the adjacent bar left if it. In the same way, thegamma-distribution reads the percentage of fibers in ordinate 43 in therange indicated on the abscissa 44 as explained above.

Another length distribution 45 is shown in FIG. 4, indicated with whitebars, which is a length distribution of the curved fibers of a set ofshort metal fibers, having an equivalent diameter of 12 μm. The set ofshort metal fibers was provided using bundle drawn stainless steelfibers, alloy AISI 316L. A representative and randomly chosen sample of242 fibers was taken. This length distribution accords to agamma-distribution 46, which is characterized with a shape factor Sbeing 3.72. An average length Lc of the curved fibers of 572 μm wasmeasured.

A set of short metal fibers as of FIG. 3, was used to improve theresistance to thermal cracking and thermal shocks of a ZrO₂—MgO basedceramic glue.

A ceramic material, being a ceramic paste, which may be used as ceramicglue, was prepared using 77 gram ZrO₂—MgO based compound and 10 gram ofwater. An amount of a set of short metal fibers having an averageequivalent diameter of 22 μm, of which the length distribution isprovided as indicated with 45 in FIG. 4, is mixed in this ceramic paste,as indicated in Table I.

The ceramic paste was heated to a temperature of 600° C., and thistemperature was kept for 90 sec. after which it was cooled to ambient in60 sec. the number of cracks on an equal surface was counted, and isresumed in Table II.

TABLE II Temperature resistant matrix Set of short % of weight of(ceramic matrix) metal fibers of short metal Number of (gram) 12 μm(gram) fibers (%) cracks (−) 77 0 0 20 77 2 2.5 16 77 4 4.9 8 77 8(sample I) 9.4 0 77 8 (sample II) 9.4 2

An identical result was obtained using a set of short metal fibers of 22μm equivalent diameter.

1. A temperature resistant material, comprising a temperature resistantmatrix and a set of short metal fibers, wherein said set of short metalfibers represents at least 0.5% of weight of said temperature resistantmaterial, said set of short metal fibers having an equivalent diameter Din the range of 1 to 150 μm, said set of short metal fibers comprisescurved fibers and entangled fibers, said curved fibers having an averagelength Lc in the range of 10 to 2000 μm, said entangled fibers having anaverage length Le, said Le being more than 5 times said Lc, wherein atleast some of said entangled fibers have a major axis which changesseveral times in a non-uniform manner.
 2. A temperature resistantmaterial as in claim 1, wherein at least 10% of said short metal fibersare entangled fibers.
 3. A temperature resistant material as in claim 1,wherein the lengths of said curved fibers are distributed according to agamma-distribution.
 4. A temperature resistant material as in claim 1, Lbeing an average length of fibers in said set of short metal fibers,wherein L/D is larger than
 5. 5. A temperature resistant material as inclaim 1, wherein Lc/D is larger than
 5. 6. A temperature resistantmaterial as in claim 1, said short metal fibers being stainless steelfibers.
 7. A temperature resistant material as in claim 1, saidtemperature resistant matrix being a ceramic matrix or ceramic glue. 8.A temperature resistant material as in claim 1, said curved fibershaving a major axis, said major axis having a direction, said directionchanging more than 90° for at least 40% of said curved fibers.
 9. Atemperature resistant material as in claim 1, said temperature resistantmaterial being a ceramic material.
 10. A temperature resistant materialas in claim 1, said temperature resistant material being a ceramic glue.11. A temperature resistant material as in claim 1, said temperatureresistant material being a temperature resistant glue.
 12. A temperatureresistant material as in claim 10, said ceramic glue comprising aceramic matrix based on SiO₂, Al₂O₃, ZrO₂ and/or MgO.
 13. A temperatureresistant material as in claim 11, said temperature resistant materialcomprising a matrix based on SiO₂, Al₂O₃, ZrO₂ and/or MgO.
 14. Thetemperature resistant material as in claim 1, comprising up to 20% byweight of short metal fibers.
 15. The temperature resistant material asin claim 1, comprising up to 15% by weight of short metal fibers. 16.The temperature resistant material as in claim 1, comprising less than10% by weight of short metal fibers.
 17. The temperature resistantmaterial as in claim 16, wherein the amount of short metal fibers is inthe range of 1% to 9.5% by weight.
 18. The temperature resistantmaterial as in claim 1, wherein said entangled fibers have an averagelength larger than 200 μm.
 19. The temperature resistant material as inclaim 18, wherein said entangled fibers have an average length largerthan 1000 μm.